Light emitting diodes (LEDs) are expected to be applied in the fields of illumination and display devices due to the advantages of small-size, low-power-consumption, and long-operating-life. Especially, in recent years, high-brightness and high-efficiency LEDs for car-mounted headlights and domestic lighting are desired. Generally, the total luminous efficiency of an LED is expressed by a product of the internal quantum efficiency and the light extraction efficiency thereof. The internal quantum efficiency is a rate of electron-hole pairs, which generate light of a target wavelength, to that of generated electron-hole pairs and is affected by the crystalline quality of a material and the presence or absence of defects. On the other hand, the light extraction efficiency is the rate of externally extracted light energy to generated light energy and is affected by the loss of light energy caused at the interface between the LED and air. Generally, the light extraction efficiency is poor, as compared with the internal quantum efficiency. Thus, in the development of high-brightness LEDs, mainly the enhancement of the internal light extraction efficiency is performed.
A hitherto proposed method for enhancing the light extraction efficiency has successfully achieved the enhancement of the light extraction efficiency by forming a nano-scaled relief structure on a surface of an LED to prevent the reflection of light at the interface between the LED and air utilizing scattering/diffraction effects. It is required to form such a relief structure as to have a size shorter than the light emission wavelength of the LED. Typical methods of forming such a relief structure are an electron beam lithography method, a nano-imprinting method, and a processing method utilizing the self-assembly of a material. Especially, the processing method utilizing the self-assembly of a material has advantages in that an LED is enabled to have a large area, and that no large equipment is necessary to form the relief structure, and that the cost for forming such a relief structure is low. Thus, the processing method of forming a relief structure utilizing the self-assembly of a material attracts attention as a useful method for enhancing the brightness of an LED.
The processing method utilizing self-assembly can realize a nano-scaled relief structure by utilizing a dot pattern formed in a microphase-separated structure of a block copolymer or a dot pattern of self-aligned nanoparticles as an etched mask. An example of utilization of the microphase-separated structure of the block copolymer is disclosed in JP-B2-4077312. An example of utilization of the self-aligned nanoparticles is disclosed in JP-A-2006-261659. However, in the case of using a block copolymer, usually, dot patterns, whose inter-dot distances are equal to or less than 100 nanometers (nm), can easily be formed to be highly separated. On the other hand, in the case of forming dot patterns whose inter-dot distances are more than 100 nm and are equal to or less than the wavelength of used light, it is necessary to use high-molecular-weight block copolymers and to perform a heat treatment for a sufficient time. Thus, the processing method utilizing self-assembly has a problem in that in a case where a finite heat treatment time is insufficient, a dot pattern, in which dots are connected to each other, is formed due to insufficient phase separation.
As is understood from the foregoing description, the method for enhancing the light extraction efficiency of an LED using a nano-scaled relief structure formed by utilizing the self-assembly of block copolymers has the following problems. That is, it is difficult to form a dot pattern, in which dots are completely separated (or isolated) from one another, on the entire light extraction surface. Thus, the light extraction efficiency is lowered due to the connection among convex portions.